This invention relates generally to monitoring liquid fuel inventory in a tank from which liquid fuel is dispensed, as via a metering means; and more particularly, it concerns monitoring both the metering means and the level of liquid fuel in the tank to enable detection of loss of liquid fuel from the tank, as from leakage or pilfering. The invention is especially adapted to underground tanks from which leakage to the exterior is not readily detectable.
During the time that liquid fuel is not dispensed from an underground tank, leakage can be detected by observing changes in the liquid top surface level in the tank. However, if liquid fuel is being intermittently dispensed from the tank, as via a pump and metering means, it is clear that changes in top surface level liquid fuel in the tank do not per se reflect leakage. There is need for accurate, reliable and simple means to monitor both liquid level in the tank as well as the amount of liquid dispensed from the tank, as via the pump and metering means, in order to arrive at an accurate determination of any such "unwanted loss". The latter can arise for example from drainage through small holes in the tank, or from pilferage from the tank, as via an inlet to the tank, or pilferage from hoses or lines leading from the tank to the dispenser.